1. Field of the Invention
The present invention relates to a ceramic heater, and more particularly to a ceramic heater applied to a glow plug used, for example, to accelerate startup of a diesel engine or applied to, among others, a heater used to ignite a kerosene fan heater.
2. Description of the Related Art
By virtue of its high strength at room temperature as well as at high temperature and small coefficient of thermal expansion, a silicon nitride ceramic heater is widely used in a glow plug or a like device. FIG. 7 shows an example of a silicon nitride ceramic heater 72 for use as a glow plug. The ceramic heater 72 is configured such that a turned (U-shaped) heating element (hereinafter also referred to as a heating element) 76 formed of electrically conductive ceramic is embedded in a ceramic substrate 75 formed of silicon nitride ceramic at a portion biased toward a front end 72a. Junction wires 78 and 79, which are formed of a high-melting-point metal, such as tungsten or molybdenum, each have one end connected to a corresponding end portion 76c (corresponding leg end portion) of the U-shaped heating element 76. The remaining end portions of the junction wires 78 and 79 are exposed on the side surface of the ceramic heater 72 in the vicinity of a rear end 72c of the ceramic heater 72, thereby serving as a pair of lead wire connection terminals (hereinafter also referred to as terminals) 81. A metallization layer (not shown) is formed on the surface of the ceramic substrate 75 in the vicinity of the lead wire connection terminals 81. Lead wires 15 are jointed to the corresponding terminals 81 by use of an Ag-based active brazing metal. This is a general joint structure for the ceramic heater 72.
In order to meet demand for a reduction in size, the ceramic heater 72 itself is shortened, with a resultant reduction in the distance between the front end 72a and lead wire joints where the lead wires 15 and the lead wire connection terminals 81 are connected. Thus, for the case where the ceramic heater 72 is installed as a glow plug in a subsidiary chamber of an engine, the temperature of the lead wire joints (hereinafter also referred to as joints) was once 200xc2x0 C. at the highest, but in recent years the lead wire joints have been exposed to a high temperature of 300xc2x0 C. or higher.
3. Problems to be Solved by the Invention
However, exposure of the joints to such high temperature has raised the following problem. Namely, a problem arises in a conventional joint structure using an Ag-based brazing metal in that the joint between a lead wire and a lead wire connection terminal suffers separation (unjoining), which is considered to be caused by migration.
One measure for coping with the problem is, for example, to impart a high melting point to an Ag-based brazing metal by employing an Ag rich composition so as to enhance heat resistance of lead wire joints. However, since a glow plug is exposed to severe heat cycles in the course of use, in order to ease generation of thermal stress in ceramic caused by a difference in thermal expansion coefficient between ceramic and an Ag-based brazing metal, such a joint structure is desirably configured such that copper, which is easily deformable, is present in the form of a buffer plate at an intermediate portion of a layer of brazing metal (hereinafter also referred to as a brazing metal layer). The joint structure is not compatible with an Ag-rich composition, for the following reason. An Ag-rich composition induces a eutectic reaction between Ag and copper; thus, a buffering effect cannot be expected. Also, use of a nickel buffer plate is not compatible with Ti contained as an activation metal in a brazing metal and thus is not applicable to the joining work. If Ti is contained in a brazing metal, Ti reacts strongly with Ni to form a layer of an intermetallic compound, thereby impairing joining strength.
Further, a technique has been proposed for preventing migration in joining by use of an Au-based brazing metal, which contains a predominant amount of gold (Au). However, this technique fails to meet the demand for reduction in cost. Further, few combinations of an Au-based brazing metal and an activation metal to be contained therein improve wettability in brazing to ceramic. Therefore, joining by use of an Au-based brazing metal is not practicable.
The prevent invention has been accomplished in view of the above-described problems, and an object of the invention is to provide a joint structure which does not impair joining strength induced by exposure to heat cycles, does not increase cost, and does not cause migration.
The above-described object has been achieved in a first aspect of the invention by providing a ceramic heater comprising a heating element embedded in an insulating ceramic substrate, and a lead wire joined to a lead wire connection terminal (electrode leading-out portion), which is connected to the heating element while electrical continuity is established therebetween, by means of a brazing metal which contains a predominant amount of copper.
A brazing metal which contains a predominant amount of copper exhibits excellent migration resistance and can retard generation of residual stress stemming from the difference in thermal expansion between electrically conductive ceramic and a lead wire, by virtue of copper""s easy deformability, thereby exhibiting only slight impairment in joining strength even upon exposure to heat cycles. Therefore, the ceramic heater of the present invention, in which lead wires are joined to lead wire connection terminals by use of such a brazing metal, can assume a joint structure which is free from the occurrence of migration without an increase in cost. As a result, the ceramic heater can assume a joint structure of high durability, heat resistance, and reliability.
In order to utilize such characteristics of copper, in a second aspect of the invention, preferably, the brazing metal contains copper in an amount of not less than 85% by mass. Also, in a third aspect of the invention, preferably, the brazing metal contains Ti or Si as an activation metal to thereby avoid the necessity of forming a metallization layer. Si effectively enhances wettability in brazing to metal or ceramic. However, a brazing metal which contains a large amount of Si suffers low ductility in the course of production thereof. In view of these phenomena, preferably, Si is contained in an amount of 0.1-5% by mass. Ti effectively enhances wettability in brazing to ceramic and contributes most to enhancement of wettability. However, when the Ti content is excessive, a brazing metal layer formed by joining exhibits increased hardness and thus becomes brittle. In view of these phenomena, in a fourth aspect of the invention, preferably, the Ti or Si content of the brazing metal is 0.1-5% by mass.
A fifth aspect of the invention is directed to the ceramic heater as described in any one of the first through fourth aspects, wherein a pad is formed on the lead wire so as to serve as a joining surface to be joined to the lead wire connection terminal, the lead wire being joined to the lead wire connection terminal via the pad. Joining via such a pad is particularly preferred when a lead wire has a circular cross section, since reliability of joining is enhanced. Notably, the pad may be formed of an Fexe2x80x94Ni alloy plate, an Fexe2x80x94Nixe2x80x94Co alloy plate, an Ni plate, or a like plate and welded to an end portion of a lead wire. Alternatively, an end portion of a lead wire may be rolled into a planate or flat shape.
In a sixth aspect of the invention, the thickness of a layer of the brazing metal is 30-400 xcexcm. This thickness range of the brazing metal layer is suited for reducing residual stress in ceramic by absorbing the difference in thermal expansion between ceramic and a lead wire as observed after joining, by utilizing the of easy plastic deformability of copper. The lower limit of the thickness range is far thicker than the thickness of a brazing metal layer in joining by use of an Ag-based brazing metal, for the following reason. Since a copper brazing metal exhibits high viscosity even near its melting point, a thin layer of copper brazing metal tends to suffer generation of pores due to insufficient spread of the brazing metal over the interface of joining, potentially resulting in insufficient joining strength. A peripheral portion of the brazing metal layer is particularly prone to this problem. However, employing a large thickness of not less than 30 xcexcm increases the amount of liquid phase at the time of melting, to thereby avoid the problem.
As discussed above, since copper exhibits easy plastic deformation, copper effectively retards, through deformation thereof, generation of residual stress in ceramic stemming from the difference in thermal expansion between ceramic and a lead wire. However, when the thickness of a brazing metal layer is less than 30 xcexcm, copper becomes less deformable, and the effect of retarding generation of residual stress cannot be expected. By contrast, since the thermal expansion coefficient of copper is far greater than that of ceramic, preferably, the thickness of a brazing metal layer is not in excess of 400 xcexcm. When the thickness of a brazing metal layer (a brazing metal layer which contains a predominant amount of copper) exceeds 400 xcexcm, thermal stress generated in the brazing metal layer becomes too large to yield a buffering effect through deformation of copper. The thus-generated large stress acts on the interface of joining with ceramic, potentially causing unjoining.
More preferably, in a seventh aspect of the invention, the thickness of a layer of the brazing metal is 50-300 xcexcm. Far more preferably, in an eighth subject of the invention, the thickness of a layer of the brazing metal is 150-250 xcexcm.
A ninth aspect of the invention is characterized in that an interjacent buffer plate formed of copper is present in a layer of brazing metal to join the lead wire and the lead wire connection terminal, and the thickness of the layer of brazing metal includes that of the buffer plate. In the present invention, when a brazing metal which contains a predominant amount of copper is used with an interjacent buffer plate formed of copper, a brazing metal layer includes the buffer plate.